1. Field of the Invention
The present invention relates to a method for synthesizing lithium titanium oxide-based anode active material nanoparticles, and more particularly, to a method for synthesizing lithium titanium oxide-based anode active material nanoparticles using a supercritical fluid condition.
2. Background of the Invention
At present, lithium secondary batteries are widely used for supplying power of information-related devices or communication-related devices such as computers, portable phones, cameras, and the like. Furthermore, in recent years, in order to reduce the dependence on petroleum and to reduce greenhouse gas emissions, the development of a plug-in hybrid electric vehicle (PHEV) and an electric vehicle, which are green electric vehicles using lithium secondary batteries as an energy source, have been competitively carried out. In addition to the transportation applications, the researches and development of lithium secondary batteries have been actively carried out as the use of medium or large-sized batteries is expected to be drastically increased in various fields such as a robot, a backup power, a medical device, a machine tool, an uninterruptible power supply (UPS), and the like. Large-sized lithium secondary batteries, particularly for use in an electric vehicle, a machine tool, an uninterruptible power supply, and the like, should charge or discharge electricity at a high rate for a short period of time. Thus it is important to develop lithium secondary batteries having a high-rate charging and discharging capacity and having a long-term stability for the large-scale applications.
At present, various carbon-based porous materials including synthetic graphite, natural graphite, hard carbon, and soft carbon, which are capable of insertion/secession of lithium ions, are widely used for an anode material of the lithium secondary battery. A carbon-based anode material has an advantage of providing an operating voltage similar to that of lithium metal, having a very stable structure, allowing long reversible charge and discharge of lithium, and having an excellent durability. However, the carbon-based anode material has a low density of graphite (theoretical density of graphite: 2.2 g/cm3), thereby having a low energy density per unit volume of the battery. Furthermore, since the oxidization and reduction potential thereof is lower by about 0.1 V than that of Li/Li+, it can be decomposed due to the reaction with an organic electrolyte used in the battery cell construction, and reacted with lithium to form a layer (hereinafter, a solid electrolyte interface (SEI) film) covering surface of the carbon material, thereby deteriorating the charge-discharge capacity. In particular, in the application field that requires a high rate discharge capacity, such as electric vehicle and the like, the formation of a SEI film deteriorating the high rate characteristic because a resistance of lithium insertion/deinsertion increases. In addition, when charged and discharged at a high rate, lithium with a high reactivity can form on the surface of carbon-based anode surface, which can react with an electrolyte or an cathode material and thus it is likely to be exploded, thereby causing a problem of deteriorating stability. Accordingly, there is an increased necessity to develop a new anode active material having high stability and reliability as well as having high performance for the large-scale applications.
In recent years, for such a anode active material of the large-sized secondary battery, lithium titanium oxide (LTO)-based anode active materials, having high stability and reliability as well as high performance, are widely used. LTO is a compound expressed by the following formula (1).LixTiyO4(0.5≦x≦3;1≦y≦2.5)
For the LTO, for example, there are Li0.8Ti2.2O4, Li2.67Ti1.33O4, LiTi2O4, Li1.33Ti1.67O4 (or Li4/3Ti5/3O4), Li1.14Ti1.71O4, and the like. Of them, the chemical formula of Li4/3Ti5/3O4 having a spinel structure during lithium charging and discharging is as follows.Li4/3Ti5/3O4+Li++e−Li7/3Ti5/3O4 
The operating voltage of Li4/3Ti5/3O4 is 1.3-1.6 V, which is higher than that of the existing carbon-based anode active material, and a volume change caused by the transformation of a spinel structure (Li4/3Ti5/3O4) to a rock-salt structure (Li7/3Ti5/3O4) is very small, which is less than 0.1%, thereby having a very excellent cycling performance. Furthermore, there exists almost no irreversible reaction, and thus it may have an advantage that above 90% of the initial capacity can be maintained even after charging and discharging for a long period of time. Furthermore, the oxidization and reduction potential of Li4/3Ti5/3O4 is high with respect to Li/Li+ (˜1.5 V) and thus an electrolyte is not almost likely to be decomposed, thereby having a very low possibility of forming a SEI film that has been a problem in the carbon-based anode material. For another advantage of Li4/3Ti5/3O4, due to a high oxidation/reduction potential, metal-type lithium that has been a problem of carbon-based anode materials during high-rate charging and discharging is not likely to be extracted, thereby having high stability during high-rate charging and discharging, and as a result, it can be used as a power source of PHEV, an electric vehicle, a machine tool, and a uninterruptible power supply. Moreover, the theoretical density thereof is about 3.5 g/cm3, which is much higher than that of the carbon-based anode material, and thus the capacity per volume is similar to the existing carbon material. Accordingly, due to the high stability, the high charge and discharge capacity and the reliability at a high rate, Li4/3Ti5/3O4 is one of promising anode materials for use in the large-sized secondary battery such as an electric vehicle and an uninterruptible power supply.
For the presently known method of synthesizing Li4/3Ti5/3O4 with a spinel structure, there is a solid-state method in which TiO2 particles and a lithium precursor such as LiOH, Li2CO3, LiNO3, and the like are mixed in a solid state at temperatures between 400 to 600° C. for 12 to 24 hours using a ball mill process or the like, and then calcinated at temperatures between 800 to 1000° C. for 12 to 24 hours. However, the solid-state method requires performing a ball mill process for a long period of time at high temperature to synthesize a single-phase material in a solid state from a mixture including a lithium precursor and TiO2, and a long calcination step is necessary to enhance the crystallinity of particles. Thus the solid-state method for producing Li4/3Ti5/3O4 is unproductive and uneconomic and typically requires large energy consumption. The formed particles have a relatively large size in a micron unit (10-100 μm; the Brunauer-Emmett-Teller (BET) specific surface area 2-5 m2/g) due to the long calcination step at high temperature and thus the insertion/deinsertion rate of lithium ions is very low, thereby causing a problem that the charge and discharge capacity is low at about 70% of its theoretical capacity. In order to increase the charge and discharge capacity of Li4/3Ti5/3O4, there has been proposed a method of synthesizing the material Li4/3Ti5/3O4 in a nano size to reduce the insertion/deinsertion distance of lithium ions within the particle. However, high energy is consumed to synthesize nanosized particles with a method of ball mill or the like, and a separate mechanical grinding process for a long period of time is necessary, thereby decreasing productivity, widening the size distribution of particles, and incurring a loss of material during a distributing process.
For other methods of synthesizing the anode active material Li4/3Ti5/3O4 having a spinel structure, there exist a vapor-based method such as spray-pyrolysis, laser ablation, plasma arc synthesis, etc. and a solution-based method such as hydrothermal method, co-precipitation, emulsion-drying, sol-gel method, etc. In case of the vapor-based method, it may be effective in synthesizing single-phase metal oxide particles having a relatively uniform particle distribution, the vapour pressure of different raw materials is different when synthesizing a composite metal oxide such as Li4/3Ti5/3O4, thereby having difficulty in controlling composition of synthesized particles. Therefore, it has a problem that an excessive quantity of impurities is highly likely to be co-produced and be mixed with Li4/3Ti5/3O4. In case of the solution-based method, composition control may be easier than the vapor-based method since it is synthesized from a uniform mixture but the size of synthesized particles is large and the distribution of particles is wide. Therefore, when applied to the synthesis of Li4/3Ti5/3O4, the solution-based methods can have a problem that the charge and discharge capacity is remarkably deteriorated. Furthermore, most of the solution-based methods require long reaction times of 12 to 48 hours up to the formation of a lithium titanium oxide-based precursor material, but lithium composition is not completely contained in the structure of the lithium titanium oxide-based precursor material, and thus it may require long calcination times for 12 to 48 hours to increase the diffusion of lithium and to increase crystallinity. Moreover, each process for synthesizing Li4/3Ti5/3O4 such as a precipitation process, a reaction process, a calcination process, and the like mostly are carried out in a batch mode, thereby having a problem that the uniformity and productivity of the product can be deteriorated. In addition, waste acids and organic solvents are produced as byproducts, causing environmental pollution.
On the other hand, a method of continuously synthesizing nanosized metal oxide using supercritical fluids was released in the beginning of the 1990s (T. Adschiri; Y. Hakuta; K. Sue; K. Arai, Journal of American Ceramic Society, 1992, 75 1019). A supercritical fluid is a fluid in a state above the critical temperature and critical pressure of the fluid, exhibiting both gas and liquid phase properties, and thus it is light in weight and very excellent in mass transfer and heat transfer as in a gas phase, and also has a property of dissolving other materials as in a liquid phase. Supercritical water (critical temperature=374° C.; critical pressure=221 bar) is one of most widely used supercritical fluids in synthesizing metal oxide nanoparticles. In the synthesis of metal oxide nanoparticles using a supercritical water, the crystalline growth rate in the supercritical water is very fast, allowing the metal oxide to be synthesized at a fast rate (<1 minute), and the solubility of reaction intermediate in the supercritical water is remarkably lower than that of water at ambient condition, allowing the metal oxide to be synthesized in a nano size, and continuous synthesis of metal oxide in supercritical water processes can be easily implemented, thereby having high productivity as well as providing a uniform size distribution of the synthesized metal oxides. Even in case of synthesizing a metal oxide using alcohol at its supercritical state such as supercritical methanol (critical temperature=240° C.; critical pressure=79 bar), supercritical ethanol (critical temperature=241° C.; critical pressure=63 bar) supercritical propanol (critical temperature=264° C.; critical pressure=52 bar), supercritical butanol (critical temperature=290° C.; critical pressure=44 bar), supercritical pentanol (critical temperature=315° C.; critical pressure=39 bar), and the like among supercritical fluids, it may have an advantage such as fast crystalline growth rate, nanosized metal oxide synthesis, high purity, and continuous process as in case of synthesizing a metal oxide using the supercritical water.